the 37 rear will give a 200lb. 8 more mph top end and will make you .200 slower in the 1/8 mile amd .400 slower in the 1/4 mile.40r is a 2.50 ratio and the 37r is 2.31.every tooth down in the rear is 2.66 mph.

for my weight 1 tooth in the front is 5.4 mph about a bike lenght per tooth in the front. the front is a lot cheaper way to go.go to side winders web site to get sprocket ratios.with the 37r on the stock quad it did 73 for me. and with the k&n filter on back of the carb i gained 3mph(2hp),so 37 r and k&n is 11 mph gain. this is also before jetting.this is what it did for me. i have been playing with different things for 2 years now.37r and 17f and ron wood stage 3 with out the nitrous it will run 102 on pavement, but you wont keep clutches if you gear like this for long.

 

sorry about that .stock with a 37r it did 81mph